It looks like it's a sliding plate, but there is a great possiblity that in wide spread the plates are bolted on. That's not oncommon on these heavy machines, so the narrow position is only for transport, and the excavator cannot be used in this position.
You should considder this option then as useful or not, if you are going to use the excavator for what it is build, namely excavating, I think it's an option that is sensitive to malfunction. If you are going to dig in the sand of course, because that kills the undercarriage.You are building some strong tracks, they will hold for shure.
Here's a picture where you can see the undercarriage is on the tracks and not beside:
Transport in wide configuration:
This one looks like a one piece undercarriage, not so strange:
If a machine is running on closed ground for it's whole life, why have an extentable that's more fragile and sensitive to malfunctions. And in the wide configuration you can still transport the machine, it's not a machine that is running jobs for a few days and then transported again.